What Nazi action marked the final stage of the "Final Solution"?

The Final solution was the act of extermination in mass numbers of anything deemed lesser by the Nazi regime, such as Jews, Poles, Russians, etc. They began to send them to death camps, and concentration camps to kill in mass numbers, as they began to lose the war. (Many will say because the Nazi's just wanted to rid of them, but it was really because in 1942 Operation Barbarossa had just failed, and they wanted to cause as much damage before losing the war, to leave Europe a hot mess to cause more conflict.)